Freeform drawing gives artists complete freedom, but is rather difficult to master. In contrast, a technique commonly known as “under painting” provides a target image as visual guidance and allows users to paint or draw over it. This method has been widely adopted by novice users as a means to improve their drawing skills, and to generate a plausible drawing that otherwise they cannot achieve. It has also been extensively used by professionals to improve the composition and layout accuracy of their art work derived from the image.
One popular drawing style is line drawing, i.e., using abstract strokes to depict the shapes of objects in a scene. When drawing over an image, the user usually wants input strokes to be aligned with object boundaries for drawing accuracy. However, accurate tracing is easy to achieve only when it is operated under direct manipulation and an input device is accurate, e.g., tracing a printed photo with a pencil. Due to the gap between the control space and the display space, indirect interaction makes it difficult for the user to accurately express his/her intention when drawing a stroke, even if the input device (e.g., a computer mouse or even a graphic tablet like Wacom Intuos) itself is of high precision. On the other hand, when the input device is inherently inaccurate (e.g., existing touch devices), it is extremely difficult for the user to specify strokes that are accurate enough for a high quality sketch.
A stroke refinement technique that automatically refines the location, orientation, and shape of input strokes by exploiting gradient features from the underlying image being traced can be used to adjust user strokes with respect to image gradients (i.e., edge snapping). A desired image edge that the user intends to trace is often mixed with other, possibly much stronger, edges in complex image regions and is difficult to extract even with the state-of-the-art edge detectors. Although various approaches have been proposed for tackling this problem, they all face the danger of snapping to a wrong edge.
The above-described background relating to line drawing techniques is merely intended to provide a contextual overview of some current issues, and is not intended to be exhaustive. Other contextual information may become further apparent upon review of the following detailed description.